Alzheimer disease (AD) affects 15 million people worldwide. Disease-modifying medications do not yet exist for AD despite widespread study of both pathologic markers of AD, [unreadable]-amyloid plaques and neurofibrillary tangles (NFTs) composed of tau. It is the goal of this application to further elucidate an early event in AD progression identified by this laboratory, the cleavage of tau to a toxic 17 kDa fragment. We hypothesize that this 17 kDa tau fragment is present in the brains of patients suffering from AD and other diseases of tau dysregulation, called tauopathies. Further, we hypothesize that this fragment exerts its toxicity via disturbance of the microtubule network and altered tau aggregation characteristics. The formation and effects of this fragment have been well characterized in primary neuronal cultures, but work is yet to be done to uncover whether the 17 kDa tau fragment is found in brain tissue of patients with AD. Obtaining this information is the focus of Specific Aim 1. Furthermore, we will assess to what extent this tau fragment is unique to the pathogenesis of AD, or whether it also plays a role in other tauopathies. In Specific Aim 2, we will establish what changes accompany, and potentially cause, the differences in the amount of 17 kDa tau fragment obtained from patients with AD and other tauopathies as compared to control subjects. Because results obtained by our laboratory have identified calpain as the protease which cleaves tau to produce the 17 kDa fragment in cultured hippocampal neurons, we will determine calpain activity in brains of patients with AD and other tauopathies. Additionally, we will assess the origin of differences in calpain activity. Finally, in Specific Aim 3 we will characterize the mechanisms underlying toxicity caused by the 17 kDa tau fragment. We will determine the role of 17 kDa tau in aggregation, be it enhancement of the formation of NFTs or polymerization into a novel oligomeric species. We will also evaluate the effect of the 17 kDa tau fragment on microtubule stability, potentially through the altered binding to full-length tau. Finally, we will assess the role of the 17 kDa tau fragment in impairment of microtubule based organelle transport. We will accomplish these aims by using the following methodologies: Western blot analysis, fluorometric enzyme activity assay, RT-PCR, primary hippocampal neuronal culture, recombinant protein expression and purification, fluorescence spectroscopy, electron microscopy, protein binding and stability assays, and time-lapse fluorescence microscopy. In summary, upon completion of these aims, we will better understand how the 17 kDa tau fragment is produced and the mechanisms underlying its toxicity. In addition, we will gain insight into the clinical relevance of this neurotoxic fragment in AD [unreadable] [unreadable]